Arrayed waveguide grating multiplexer-demultiplexer and related control method

ABSTRACT

An arrayed waveguide grating multiplexer/demultiplexer includes an array of optical waveguides ordered in sequence from a shortest waveguide up to a longest waveguide, and identical phase shifters configured to be controlled by a same control signal. Each phase shifter increases/decreases an optical path of an optical waveguide by the same quantity based on the control control signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of pending U.S. PatentPublication No. 2015/0309258 published Oct. 29, 2015, which claimspriority to IT Application No. MI2014A000787 filed Apr. 29, 2014, all ofwhich are hereby incorporated herein in their entireties by reference.

FIELD OF THE INVENTION

The present disclosure relates to electro-optical devices, and moreparticularly, to a wavelength division multiplexer or demultiplexerbased on arrayed waveguide gratings, and to a method of controlling anarrayed waveguide grating multiplexer or demultiplexer.

BACKGROUND

Transmission systems on optical fiber predominantly use predeterminedwindows (i.e., bands, channels) of the optical spectrum through whichthe transmission of the signals along the fibers takes place withminimum attenuation. Signals or communication channels, each with itsown precisely defined wavelength as produced by a relevant lasergenerator, included in one of these windows or bands may be transmittedalong an optical fiber with extremely low losses. The simultaneoustransmission of various communication channels belonging to a certainband, window or channel on a same fiber is made possible by WavelengthDivision Multiplexing (WDM).

Arrayed Waveguide Gratings (AWG) are devices capable of multiplexing aplurality of optical signals at different wavelengths into a singleoptical fiber, and demultiplexing optical signals at differentwavelengths transmitted over a single optical fiber. As a result of thisproperty, they may be used in particular as wavelength divisiondemultiplexers to retrieve individual channels of different wavelengthsat the receiving end of an optical communication network.

A schematic diagram of an arrayed waveguide grating (AWG) demultiplexeris shown in FIG. 1. It substantially comprises a first slab waveguide 1defining a first free propagation region (FPR), a second slab waveguide2 defining a second free propagation region coupled to the first slabwaveguide 1 through an array of optical waveguides 3. The first slabwaveguide 1 is coupled to receive multiplexed optical signals ofdifferent wavelengths λ₁, λ₂, λ₃, λ₄, for example, conveyed through afirst optical waveguide 4, and to irradiate them towards first endportions of the waveguides 3. The optical waveguides of the array defineoptical paths of different lengths. More precisely, each waveguide ofthe array is shorter by the same fixed length ΔL than the longeradjacent waveguide. Each waveguide is longer by a fixed length ΔL thanthe shorter adjacent waveguide, except the shortest waveguide.

When the optical signals have crossed the arrayed waveguides, they reachthe second end portion thereof from which they are irradiated throughthe second free propagation region of the second slab waveguide 2.Optical signals of a same wavelength constructively interfere with amaximum intensity at a respective main focal spot located in a positionthat depends on the wavelength, as shown in FIG. 1.

This device is sensitive to temperature variations or process spread. Asschematically shown in FIGS. 2a and 2b , temperature variations maychange the optical path difference n_(eff)·ΔL between adjacentwaveguides, with n_(eff) being the effective refractive index of thewaveguides. As a consequence, the focal spots of the demultiplexedoptical signals may be shifted clockwise or counter-clockwise dependingon the variation of the effective refractive index n_(eff) upontemperature. To prevent information losses, the number of opticalsignals at different wavelengths that may be transmitted on a sameoptical fiber is smaller than the maximum number that in theory could beallowed in absence of temperature variations.

An AWG device is disclosed in the article by Andrew Hang, Cary Gunn,Guo-Liang Li, Yi Liang, Sina Mirsaidi, Adithyaram Narasimha, ThierryPinguet, “A 10 Gb/s photonic modulator and WDM MUX/DEMUX integrated withelectronics in 0.13 μm SOI CMOS”, ISSCC 2006, Session 13, Opticalcommunication, 13.7. This prior AWG has one single array of identicalPIN junction phase modulators individually controlled by a dedicated DACintegrated into each arm of the AWG, and may be used to restore thephase relationship of the light due to errors in fabrication of opticalwaveguides that induce random delays to the optical signal.

In this case, the errors in fabrication may randomly effect anywaveguide and, if required, a correction needs to be appliedindividually on each arm of the AWG. The number of waveguides can easilygrow up to 100 or more, and the algorithm and the electronics dedicatedto controlling all the DACs become too complex to be practically formed.

SUMMARY

An arrayed waveguide grating multiplexer-demultiplexer, and a method ofcontrolling the same, for compensating the effect of temperaturefluctuations are provided.

The method comprises the steps of providing and coupling at least afirst phase shifter to the shortest/longest waveguide, and providing andcoupling, for each optical waveguide but the shortest/longest waveguide,a number of phase shifters identical to the first phase shifter. Thisnumber my be greater by a constant integer than the number of identicalphase shifters coupled to the longest/shortest one of thesmaller/greater optical waveguides of the array. All of the identicalphase shifters may receive a same control signal to make each phaseshifter increase/decrease by a same amount, as determined by the controlsignal. The control signal may be received over the optical path of theoptical waveguide to which it is coupled.

An arrayed waveguide grating demultiplexer suitable for implementing theabove method may differ from the prior demultiplexer of FIG. 1 becauseit comprises a plurality of identical phase shifters configured to becommanded with the same control signal. Each phase shifter may beconfigured to increase/decrease by substantially the same amount, asdetermined by the control signal. At least a phase shifter may becoupled to the shortest/longest waveguide. Each optical waveguide butthe shortest/longest waveguide may be coupled to a number of identicalphase shifters greater by a constant integer than the number ofidentical phase shifters respectively coupled to the longest/shortestone of the smaller/greater optical waveguides of the array.

According to an embodiment, the arrayed waveguide grating demultiplexermay comprise a photo-detector placed at a position located beyond anoutermost of the main focal spots of the second free propagation regionso as to not be illuminated in normal functioning conditions, and to beilluminated when the arrayed waveguide grating demultiplexer undergoes atemperature variation greater than a minimum threshold. Thephoto-detector may be configured to generate an electrical error signalcorresponding to the intensity of an optical signal impinging thereon. Acontrol block may be configured to receive the electrical error signalas an input, and to generate the control signal.

According to another embodiment, in operation, components of a samewavelength as the output optical signals may also constructivelyinterfere with a reduced intensity at a respective secondary focal spotlocated in a position of the second free propagation region depending onthe wavelength. The arrayed waveguide grating demultiplexer may comprisea plurality of photo-detectors, each placed at the position of arespective secondary focal spot and configured to generate a respectiveelectrical error signal corresponding to the intensity of an opticalsignal impinging thereon. A control block may be configured to receiveall the electrical error signals as input, and to generate the controlsignal.

According to yet another embodiment, the arrayed waveguide gratingdemultiplexer may comprise a laser source placed in correspondence of arespective main focal spot of the second free propagation region. Thelaser source may be configured to irradiate an optical reference signalfrom the second free propagation region to the first free propagationregion throughout the array of optical waveguides. In operation,components of a same wavelength of the optical reference signal mayconstructively interfere with a maximum intensity at a respective mainfocal spot located in a position of the first free propagation regiondepending on the wavelength. A plurality of photo-detectors may beplaced in correspondence of a respective one of the main focal spots ofthe first free propagation region. Each of the photo-detectors may beconfigured to generate a respective electrical error signalcorresponding to the intensity of an optical signal impinging thereon. Acontrol block may be configured to receive all the electrical errorsignals as input, and to generate the control signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of an arrayed waveguide gratingdemultiplexer according to the prior art.

FIGS. 2a and 2b illustrate exemplary effects of temperature fluctuationsin an arrayed waveguide grating demultiplexer according to the priorart.

FIG. 3 illustrates how identical phase shifters are coupled together andto the optical waveguides of the arrayed waveguide grating demultiplexerdepending on their lengths, according to the present disclosure.

FIG. 4a depicts an embodiment of the arrayed waveguide gratingdemultiplexer according to the present disclosure.

FIGS. 4b and 4c depict the arrayed waveguide grating demultiplexer ofFIG. 4a in two different functioning conditions in which the opticalpath difference of the waveguides is to be adjusted.

FIG. 5a schematically shows another embodiment of the arrayed waveguidegrating demultiplexer that exploits secondary focal spots of the secondfree propagation region according to the present disclosure.

FIG. 5b is a graph intensity-position of the main focal spots and of thesecondary focal spots of the second free propagation region of thearrayed waveguide grating demultiplexer of FIG. 5 a.

FIG. 5c shows a multi-port detector that may be used at the second freepropagation region of the arrayed waveguide grating demultiplexer ofFIG. 5 a.

FIG. 6 schematically shows yet another embodiment of the arrayedwaveguide grating demultiplexer that has a laser source for injecting anoptical reference signal from the second free propagation region back tomain focal spots of the first free propagation region according to thepresent disclosure.

DETAILED DESCRIPTION

An arrayed waveguide grating multiplexer/demultiplexer in which it ispossible to compensate with great accuracy effects of temperaturefluctuations comprises a plurality of identical phase shifters PS asschematically depicted in FIG. 3. The identical phase shifters arecommanded all together using a same electrical control signal, generatedby a control block CONTROL. With this coupling, each phase shifterincreases/decreases by a same amount the optical path of the light inthe waveguide of the array 3 to which it is coupled.

As already stated above, there is a fixed optical path differencen_(eff)·ΔL between adjacent optical waveguides of the array 3. Thus, anincrease of the effective refractive index will increase more inabsolute terms in the optical path of a longer waveguide than theoptical path of a shorter waveguide. For this reason, the number ofphase shifters PS coupled to each waveguide of the array 3 depends onthe length of the waveguide.

More precisely, the shortest/longest waveguide is coupled to at leastone phase shifter PS (only one, in the example of FIG. 3) and each ofthe other optical waveguides is coupled to a number of identical phaseshifters greater by a constant integer (1 in FIG. 3) than the number ofidentical phase shifters coupled to the longest/shortest of theshorter/longer optical waveguides.

This solution is particularly advantageous because it allows adjustmentin a very accurate fashion of the effects of temperature variations.Indeed, the phase shifters PS may be precisely realized identical toeach other. Moreover, the control signal, distributed in parallel to allphase shifters, does not undergo relevant drops along the electricallines through which it is distributed. Thus all identical phase shiftersare effectively commanded by the same signal.

Suitable phase shifters may be thermal phase modulators or electro-opticphase modulators. These devices are currently available and are wellknown to the skilled person, and for this reasons they will not bediscussed further.

FIG. 4a depicts an embodiment of the arrayed waveguide gratingdemultiplexer having two networks of identical phase shifters as shownin FIG. 3 in order to increment or decrement the optical path differencen_(eff)·ΔL between adjacent optical waveguides.

FIGS. 4b and 4c show the arrayed waveguide grating demultiplexer of FIG.4a in two different functioning conditions in which a temperaturevariation has occurred and a control block CONTROL has to adjust theoptical path difference n_(eff)·ΔL between adjacent optical waveguidesof the array 3.

Normally a phase shifter can increase or decrease the effectiverefractive index of a waveguide depending on the sign of the coefficientof the particular used effect but it cannot act in both directions. Asan example to better explain this behavior, consider a thermal phaseshifter acting on a waveguide with a negative thermo-opticalcoefficient. In this case, the phase shifter can only decrease theeffective refractive index and not increase it.

With the particular architecture depicted in FIGS. 4a and 4b and itssymmetry, it is possible to use the upper-left array to apply anequivalent incremental decrease and to use the lower-right array toapply an equivalent incremental increase of the effective index. Thisarchitecture allows in such a way both a red-shift and a blue-shift inthe spectral output of the device.

To compensate a temperature variation in a certain range, i.e., [T1,T2],it is convenient to design and realize a device working nominally at amid point of the range, T3=(T1+T2)/2. In this case, only one of the twophase shifters array may be used to compensate up to half of the totaltemperature variation, between T3 and T1 OR between T3 and T2.

If one and not two arrays of phase shifters are used it is possible toapply only a red or a blue shift and to compensate a temperaturevariation in the same range [T1,T2] it is possible to design and realizea device working nominally at T1 or at T2. An average of twice theelectrical power needs to be spent to control the same device. Theadvantage of this energy savings is clear when the device to becontrolled is used in a large data center where electrical powerconsumption and thermal control are normally important issues.

In operation, components of a same wavelength of output optical signalsirradiated throughout the second free propagation region constructivelyinterfere with a maximum intensity at a respective main focal spotlocated in a position determined by the wavelength of the output opticalsignals. In absence of temperature variations, the four multiplexedoptical signals λ₁, λ₂, λ₃, λ₄ should be received at the positions P₁,P₂, P₃ and P₄. Because of variations of the effective refractive index,the focal spots at the second free propagation region are shiftedcounterclockwise (FIG. 4b ) or clockwise (FIG. 4c ) and at least anoptical signal is not correctly received. In a standard AWG device theoptical channels are uniformly shifted, and consequently, may not becorrectly received.

In order to sense this improper functioning condition, according to anembodiment, the arrayed waveguide grating demultiplexer comprises atleast a photo-detector (DETECTOR1, DETECTOR2) placed at a positionlocated beyond an outermost of the focal spots of the second freepropagation region so as to be not illuminated in normal functioningconditions, and to be illuminated when the arrayed waveguide gratingdemultiplexer undergoes a temperature variation greater than a minimumthreshold. The photo-detector is configured to generate an electricalerror signal corresponding to the intensity of an optical signalimpinging thereon. This electrical error signal is provided as an inputto a control block CONTROL configured to generate the control signalthat commands the phase shifters PS shown in FIG. 4 a.

Preferably, there will be two photo-detectors DETECTOR1 and DETECTOR2placed respectively to sense a counterclockwise or clockwise shift ofthe focal spots of the output signals. It is helpfull to specify thatthe control signal may be applied to the first or second phase shiftarray to correct the counterclockwise or clockwise shift of the focalspots of the output signals. The two detectors may be specifically usedfor this purpose, and in conjunction with the two arrays of phaseshifters it is possible to operate a blue-shift or a red-shift of thesignals in the output ports.

According to another embodiment as shown in FIG. 5a , it is possible toexploit the fact that, in operation, components of a same wavelength ofthe output optical signals also constructively interfere with a reducedintensity at a respective secondary focal spot that is located in aposition of the second free propagation region depending on thewavelength, as schematically illustrated in the graph of FIG. 5b . Inthis embodiment, the arrayed waveguide grating demultiplexer comprises aplurality of photo-detectors DETECTORS, each placed at the position of arespective secondary focal spot and configured to generate a respectiveelectrical error signal corresponding to the intensity of an opticalsignal impinging thereon. A control block CONTROL is configured toreceive in input all the electrical error signals, and to generateaccordingly, the control signal for commanding all phase shifterstogether in order to compensate the effects of temperature variations.This embodiment may be implemented with a multi-port detector, as shownin FIG. 5c , installed at the second free propagation region of thearrayed waveguide grating demultiplexer. In this embodiment, ports 5, 6,7 and 8 are the output ports of the demultiplexer while ports 1, 2, 3,4, 9, 10, 11 and 12 are used for monitoring.

According to yet another embodiment, as illustrated in FIG. 6, thearrayed waveguide grating demultiplexer includes a laser source placedin correspondence of a respective main focal spot of the second freepropagation region. This laser source is configured to irradiate anoptical reference signal from the second free propagation region to thefirst free propagation region throughout the array of opticalwaveguides. In operation, components of a same wavelength of the opticalreference signal constructively interfere with a maximum intensity at arespective main focal spot located in a position a of the first freepropagation region FPR, moving respectively towards b or c depending ontemperature variation. By placing a plurality of photo-detectors eachplaced in correspondence of a respective one of the main focal spots aor b or c, it is possible to generate an electrical error signal thatmay be exploited by a control block (not shown in the figure) togenerate the control signal for commanding all the phase shifters ofFIG. 3.

Even if it has not been shown in detail, the skilled person willrecognize that all embodiments of the arrayed waveguide gratingdemultiplexer of this disclosure may be equipped with an input waveguideconfigured to convey multiplexed optical signals of differentwavelengths, a balanced power divider coupled to receive the multiplexedoptical signals and to irradiate them towards radiating/capturingelements defined on end portions of the optical waveguides of the array,and a plurality of output waveguides each placed in correspondence of arespective main focal spot of the second free propagation region tocollect a corresponding spectral component of the output signals.

That which is claimed is:
 1. An optical device comprising: a first slabwaveguide; a second slab waveguide; an array of optical waveguidesordered in sequence from a shortest optical waveguide to a longestoptical waveguide, having first end portions and second end portions,respectively, coupled to the first slab waveguide and the second slabwaveguide; each optical waveguide, except for the shortest opticalwaveguide, being longer by a same amount than a preceding opticalwaveguide; and a plurality of identical phase shifters; the shortestoptical waveguide or the longest optical waveguide having at least onephase shifter coupled thereto, with each of said remaining opticalwaveguides having a number of phase shifters different by a constantinteger than the number of phase shifters coupled to the opticalwaveguide that precedes or follows in the sequence so that a differencein the number of phase shifters between adjacent optical waveguides isequal to the constant integer.
 2. The optical device of claim 1, furthercomprising: a photo-detector positioned beyond an outermost focal spotso as not to be illuminated in normal functioning conditions and to beilluminated based upon a temperature variation greater than a threshold,said photo-detector being configured to generate an error signal; and acontrol block configured to generate a control signal for the pluralityof identical phase shifters based upon the error signal.
 3. The opticaldevice of claim 1, wherein in operation components of a same wavelengthconstructively interfere with a reduced intensity at a respectivesecondary focal spot located in a position depending on the wavelength;and further comprising: a plurality of photo-detectors, eachphoto-detector placed at a position of a respective secondary focal spotand configured to generate a respective error signal; and a controlblock configured to generate a control signal for the plurality ofidentical phase shifters based upon the error signals.
 4. The opticaldevice of claim 1, further comprising: a laser source placed at arespective main focal spot and configured to irradiate an opticalreference signal from the second slab waveguide to the first slabwaveguide through said array of optical waveguides, wherein in operationcomponents of a same wavelength of the optical reference signalconstructively interfere with an increased intensity at a respectivemain focal spot located in a position of the first slab waveguidedepending on the wavelength; a plurality of photo-detectors each placedin correspondence of a respective one of the main focal spots of thefirst slab waveguide, each photo-detector being configured to generate arespective error signal corresponding to the intensity of an opticalsignal impinging thereon; and a control block configured to generate thecontrol signal based upon the error signals.
 5. The optical device ofclaim 1, wherein the first end portions and second end portions define afirst array and a second array of radiating/capturing elements,respectively, to receive and to deliver optical signals.
 6. The opticaldevice of claim 5, further comprising: an input waveguide configured totransmit multiplexed optical signals of different wavelengths; abalanced power divider configured to receive the multiplexed opticalsignals and to irradiate them towards said first array ofradiating/capturing elements; and a plurality of output waveguides eachplaced in correspondence of a respective main focal spot of the secondfree propagation region to collect a corresponding spectral component ofthe output signals.
 7. The optical device of claim 1, further comprisinga control block coupled to each of said plurality of identical phaseshifters.
 8. The optical device of claim 7, wherein said control blockis configured to provide a common control signal to each of saidplurality of identical phase shifters.
 9. The optical device of claim 1,wherein said plurality of identical phase shifters comprises: a firstgroup to provide an incremental decrease in effective index of saidarray of optical waveguides; and a second group to provide anincremental increase in effective index of said array of opticalwaveguides.
 10. The optical device of claim 1, wherein each of saidplurality of identical phase shifters comprises a thermal phasemodulator.
 11. The optical device of claim 1, wherein each of saidplurality of identical phase shifters comprises an electro-optic phasemodulator.
 12. An optical device comprising: a first slab waveguide; asecond slab waveguide; an array of optical waveguides ordered insequence from a shortest optical waveguide to a longest opticalwaveguide, having first end portions and second end portions,respectively, coupled to the first slab waveguide and the second slabwaveguide; each optical waveguide, except for the shortest opticalwaveguide, being longer by a same amount than a preceding opticalwaveguide; a plurality of identical phase shifters; the shortest opticalwaveguide or the longest optical waveguide having at least one phaseshifter coupled thereto, with each of said remaining optical waveguideshaving a number of phase shifters different by a constant integer thanthe number of phase shifters coupled to the optical waveguide thatprecedes or follows in the sequence so that a difference in the numberof phase shifters between adjacent optical waveguides is equal to theconstant integer; and a control block coupled to said plurality ofidentical phase shifters; said plurality of identical phase shifterscomprising a first subarray to provide an incremental decrease ineffective index of said array of optical waveguides, and a secondsubarray to provide an incremental increase in effective index of saidarray of optical waveguides.
 13. The optical device of claim 12, furthercomprising: a photo-detector positioned beyond an outermost focal spotso as not to be illuminated in normal functioning conditions and to beilluminated based upon a temperature variation greater than a threshold,said photo-detector being configured to generate an error signal; andwherein said control block is configured to generate a control signalfor the plurality of identical phase shifters based upon the errorsignal.
 14. The optical device of claim 12, wherein in operationcomponents of a same wavelength constructively interfere with a reducedintensity at a respective secondary focal spot located in a positiondepending on the wavelength; and further comprising: a plurality ofphoto-detectors, each photo-detector placed at a position of arespective secondary focal spot and configured to generate a respectiveerror signal; and wherein said control block is configured to generate acontrol signal for the plurality of identical phase shifters based uponthe error signals.
 15. The optical device of claim 12, furthercomprising: a laser source placed at a respective main focal spot andconfigured to irradiate an optical reference signal from the second slabwaveguide to the first slab waveguide through said array of opticalwaveguides, wherein in operation components of a same wavelength of theoptical reference signal constructively interfere with an increasedintensity at a respective main focal spot located in a position of thefirst slab waveguide depending on the wavelength; a plurality ofphoto-detectors each placed in correspondence of a respective one of themain focal spots of the first slab waveguide, each photo-detector beingconfigured to generate a respective error signal corresponding to theintensity of an optical signal impinging thereon; and wherein saidcontrol block is configured to generate the control signal based uponthe error signals.
 16. The optical device of claim 12, wherein the firstend portions and second end portions define a first array and a secondarray of radiating/capturing elements, respectively, to receive and todeliver optical signals.
 17. The optical device of claim 16, furthercomprising: an input waveguide configured to transmit multiplexedoptical signals of different wavelengths; a balanced power dividerconfigured to receive the multiplexed optical signals and to irradiatethem towards said first array of radiating/capturing elements; and aplurality of output waveguides each placed in correspondence of arespective main focal spot of the second free propagation region tocollect a corresponding spectral component of the output signals. 18.The optical device of claim 12, wherein said control block is configuredto provide a common control signal to each of said plurality ofidentical phase shifters of a respective group.
 19. The optical deviceof claim 12, wherein each of said plurality of identical phase shifterscomprises a thermal phase modulator.
 20. The optical device of claim 12,wherein each of said plurality of identical phase shifters comprises anelectro-optic phase modulator.
 21. A method for making an optical devicecomprising: assembling an array of optical waveguides ordered insequence from a shortest optical waveguide to a longest opticalwaveguide, having first end portions and second end portions,respectively, coupled to a first slab waveguide and a second slabwaveguide, each optical waveguide, except for the shortest opticalwaveguide, being longer by a same amount than a preceding opticalwaveguide; and coupling at least one phase shifter from among aplurality of identical phase shifters to the shortest optical waveguideor the longest optical waveguide, with each of said remaining opticalwaveguides having a number of phase shifters different by a constantinteger than the number of phase shifters coupled to the opticalwaveguide that precedes or follows in the sequence so that a differencein the number of phase shifters between adjacent optical waveguides isequal to the constant integer.
 22. The method of claim 21, furthercomprising: positioning a photo-detector beyond an outermost focal spotso as not to be illuminated in normal functioning conditions and to beilluminated based upon a temperature variation greater than a threshold,said photo-detector being configured to generate an error signal; andgenerating a control signal for the plurality of identical phaseshifters based upon the error signal.
 23. The method of claim 21,wherein said plurality of identical phase shifters comprises: a firstgroup to provide an incremental decrease in effective index of saidarray of optical waveguides; and a second group to provide anincremental increase in effective index of said array of opticalwaveguides.